(287c) Organic Solute Transport in Liquid Crystal Membranes

Small molecule thermotropic liquid crystals (TLCs) exhibit anisotropic fluid-like phases that are responsive to temperature and to applied electric and magnetic fields. Rod?like TLC molecules tend to align along the long axis of the molecule to form nematic or smectic phases. We expect interesting transport behavior of dissolved solutes in TLCs depending on the molecular structure and solute/TLC intermolecular interactions. Prior work in the area has concentrated on the solubility and diffusivity of gas molecules in TLCs, or on qualitative assessments of TLC membranes for use in temperature-controlled drug release. However, there has been a lack of diffusivity and solubility data in the transport of organic solutes in TLCs. To quantify the transport behavior of organic solutes in the TLCs, we studied the transport of various organic solutes in aqueous environments across TLC membranes consisting of (4-cyano-4′-octylbiphenyl (8CB) and 4-(trans-4′-pentylcyclohexyl)benzonitrile (PCH5)) embedded in cellulose nitrate porous supports at different temperatures. The concentrations of organic solutes diffusing across the liquid crystal membrane were monitored by a UV spectrometer. Solute diffusivity and solubility were calculated using a time-lag method which relies on the analysis of unsteady and steady state permeation regimes across the membrane. The diffusivity and solubility depend on intra- and intermolecular hydrogen bonding, which changes the number of hydrogen bonding sites on solutes available for interactions with water molecules and the cyano group of the TLCs. Rod-like solutes, such as phenol and para-creso, had increased solubility in the nematic phase compared to the isotropic phase based on the shape selective absorption. The nematic and smectic phases showed lower activation energies for diffusion than the isotropic phase, which suggests that the randomly oriented isotropic phase is more of a barrier to diffusion.